DEV: Make user field validation more specific (#16746)

- Only validate if custom_fields are loaded, so that we don't trigger a db query
- Only validate public user fields, not all custom_fields

This commit also reverts the unrelated spec changes in ba148e08, which were required to work around these issues

spec/services/user_anonymizer_spec.rb

@@ -289,9 +289,9 @@ describe UserAnonymizer do
           "user_field_#{field2.id}": "bar",
           "another_field": "456"
         }
-        user.save
 
-        expect { make_anonymous }.to change { user.reload.custom_fields }.to "some_field" => "123", "another_field" => "456"
+        expect { make_anonymous }.to change { user.custom_fields }
+        expect(user.reload.custom_fields).to eq("some_field" => "123", "another_field" => "456")
       end
     end
   end